1. Field of the Invention
The present invention relates to the field of voltage regulators and in particular to regulators with a low drop out.
2. Description of the Related Art
A low drop out (LDO) regulator made in the form of an integrated circuit may be used to provide a predetermined voltage with low noise to a set of electronic circuits from a supply voltage provided by a rechargeable battery. Such a supply voltage decreases along time and is likely to include noise caused by the action of neighboring electromagnetic radiations on the battery-to-regulator connections. The regulator is said to have a low drop out since it enables providing a voltage close to the supply voltage.
FIG. 1 schematically shows a conventional low drop out regulator. The regulator includes an output terminal O provided to be connected to a load R. Load R, which is essentially resistive, represents the general input impedance of all the circuits supplied by the regulator. For simplicity, it is considered hereafter that load R is a resistor. The regulator includes an operational amplifier 2 having an inverting input Exe2x88x92 connected to a positive reference voltage Vref and having a non-inverting inverting input E+ connected to output terminal O by a feedback loop. Operational amplifier 2 is supplied between a positive supply voltage Vbat provided by the battery and a ground voltage GND. A P-channel MOS power transistor T1 has its drain connected to output terminal O and its source connected to voltage Vbat. The gate of transistor T1 is connected to the output terminal of inverting amplifier 2. Transistor T1 is a MOS transistor, especially to minimize, with respect to the use of a bipolar transistor, the difference between output voltage Vout of terminal O and supply voltage Vbat. A charge capacitor C is arranged between output terminal O and voltage GND.
The regulator maintains the voltage of output terminal O to a value equal to reference voltage Vref. Any variation in voltage Vbat translates as a variation in voltage Vout, which is transmitted by the feedback loop on terminal Exe2x88x92. Any variation in load R translates as a variation in current Iout provided by the regulator to the load. When load R decreases, current Iout increases. Conventionally, the voltage regulator includes a device of protection against short-circuits intended for limiting the regulator consumption by setting the maximum current that can be provided by the regulator.
The regulator includes a device 4 of protection against short-circuits. Device 4 includes a P-channel MOS transistor T2 having its source connected to the gate of transistor T1. The drain of transistor T2 is connected to the drain and to the gate of an N-channel MOS transistor 6 having its source connected to voltage GND. A current source CS generating a current Iref is also connected to the drain of transistor T2. An N-channel MOS transistor 7 has its source connected to voltage GND and its gate connected to the gate of transistor 6. Transistor 7 is connected to voltage Vbat via a resistor R1. A P-channel MOS transistor T3 has its source connected to voltage Vbat, its drain connected to the gate of transistor T1, and its gate connected to the drain of transistor 7.
Current Irep flowing through transistor T2 depends on current Iout flowing through transistor T1 due to the fact that the sources of these transistors are interconnected and that their gates receive a same signal. The current flowing through transistor 6 is null when current Irep flowing through transistor T2 is smaller than current Iref. No current then flows through transistor 7 and resistor R1, and the gate of transistor T3 has a voltage equal to Vbat. When current Irep is greater than Iref, transistor 6, transistor 7, and resistor R1 are run through by a current equal to Irepxe2x88x92Iref. The gate of transistor T3 then has a potential equal to Vbatxe2x88x92R1(Irepxe2x88x92Iref). Transistors T2 and T3, resistor R1, and current Iref are chosen so that, when current Iout is smaller than a threshold value It, transistor T3 is not on. If current Iout exceeds threshold value It, transistor T3 turns on and tends to bring the gate voltage of transistor T1 to voltage Vbat. Transistor T1 then becomes less conductive and current Iout returns to limiting value It. Circuit 4 thus enables limiting the current in the load to value It. Current It must be greater than the nominal current to be provided by the regulator.
A disadvantage of device 4 is that upon power-on of the regulator, capacitor C is charged with a current equal to current It whatever the value of resistance R. This high-current charge results in heating up and damaging capacitor C.
An object of the present invention is to provide a device of protection against short-circuits which enables avoiding for capacitor C to be run through by a strong current at the regulator power-on.
To achieve this object, the present invention provides a voltage regulator having an output terminal adapted to being connected to a load, including a device for limiting the current flowing through the load to a first threshold current if the voltage of the output terminal is lower than a threshold voltage, and to a second current threshold higher than the first current threshold if the voltage of the output terminal is greater than the threshold voltage.
According to an embodiment of the present invention, the limiting device includes a comparator for comparing the voltage of the output terminal to the threshold voltage, first and second feedback loops for limiting the current flowing through the load respectively to the first and second current thresholds, and a switching block controllable by the comparator to activate either the first or the second feedback loop according to whether the voltage of the output terminal is smaller or not than the threshold voltage.
According to an embodiment of the present invention, the switching block is adapted to providing a current depending on the current running through the load on a first or on a second output, and each feedback loop, connected to an output of the switching block, includes a control block adapted to providing a control signal when it receives from the switching block a current greater than a reference current, and further includes a turn-off means which receives the output of the control blocks and which decreases the current running through the load when any one of the first and second control signals is active.
According to an embodiment of the present invention, the voltage regulator includes a power switch arranged to connect the output terminal to a first supply voltage, and a first operational amplifier having its inverting and non-inverting inputs respectively connected to the reference voltage and to the output terminal, a control terminal of the power switch being connected to the output of the first operational amplifier and the device for limiting the current flowing through the load being connected to the control terminal of the power switch, the load including a capacitor and a first impedance connected in parallel between the output terminal and a second supply voltage.
According to an embodiment of the present invention, the switching block includes a first MOS transistor of a first type having its source connected to the first supply voltage and its gate connected to the control terminal of the power switch, and second and third MOS transistors of the first type having their sources connected to the drain of the first transistor, the drains of the second and third transistors respectively forming the first and second outputs of the switching block.
According to an embodiment of the present invention, the comparator includes fourth and fifth MOS transistors of a second type having their drains connected to the first supply voltage, having their gates respectively connected to the threshold voltage and to the output terminal, the sources of the fourth and fifth transistors being respectively connected to the gates of the second and third transistors, as well as to the second supply voltage via first and second current sources.
According to an embodiment of the present invention, the control block of each feedback loop includes a pair of MOS transistors of the second type having their sources connected to the second supply voltage, having their gates connected to each other and to a current source generating a reference current, the drain and the gate of a first transistor of the transistor pair being interconnected and connected to one of the outputs of the switching block, the current running through the second transistor of the transistor pair corresponding to the control signal provided by the control block.
According to an embodiment of the present invention, the turn-off means which receives the output of the control blocks includes a resistor having a first terminal connected to the first supply voltage and a second terminal arranged to receive the sum of the control signals provided by the control blocks, and a sixth MOS transistor of the first type having its source connected to the first supply voltage, having its drain connected to the control terminal of the power switch, and having its gate connected to the second terminal of the resistor.
According to an embodiment of the present invention, the turn-off means which receives the output of the control blocks includes a second impedance having a first terminal connected to the first supply voltage and a second terminal arranged to receive the sum of the control signals provided by the control blocks, a third impedance, matched with the second impedance, a first terminal of which is connected to the first supply voltage and a second terminal of which receives a predetermined constant current, a second operational amplifier having its non-inverting and inverting inputs respectively connected to the second terminal of the second and third impedances, and a seventh MOS transistor of the first type having its source connected to the first supply voltage, having its drain connected to the control terminal of the power switch, and having its gate connected to the output of the second operational amplifier.
According to an embodiment of the present invention, the first supply voltage, the reference voltage, and the threshold voltage are positive voltages of decreasing values, the second supply voltage is a ground voltage, the power switch and the transistors of the first type are P-channel MOS transistors, and the transistors of the second type are N-channel MOS transistors.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.